1. Field of the Invention
The present invention relates to a process for preparing polyimides. In particular, it relates to a direct process for preparing semi-crystalline polyimides.
2. Description of the Related Art
Typically, linear aromatic polyimides are prepared by reacting a dianhydride with a diamine in the presence of a polar aprotic solvent. The resulting product is a polyamic acid which is subsequently imidized to form a polyimide. Imidization can take place either through chemical dehydration or through the application of heat. The resulting polyimides have exceptional thermal and oxidative stabilities and are useful for such applications as adhesives, matrix resins for composites and molding powders.
Despite these properties, many of these polyimides are difficult to process into useful form because of their limited melt-flow properties. Most of the existing polyimides require processing temperatures approaching 400.degree. C. and pressures near 10,000 psi in order to sinter to form a well consolidated part. Thus, in order for linear aromatic polyimides to be useful as adhesives, matrix resins and molding powders, it is desired that the polymers exhibit flow at lower temperatures and pressures.
St. Clair (U.S. Pat. No. 5,061,783) sought to achieve this through providing a two-part process for converting the polyamide-acid precursors of polyimides to polyimides which exhibit maximum levels of crystallinity, so that these crystalline forms can then be melted at relatively moderate temperatures to afford adequate flow for molding and consolidation operations. This process involves first dissolving a polyamide-acid in an amide solvent, or a mixture of an ether and an amide solvent, at low percent solids, followed by treating this solution with an aprotic organic base, such as triethylamine or pyridine. The polyamide-acid employed is the reaction product of 3,3',4,4'-benzophenonetetracarboxylic dianhydride and an aromatic diamine, especially 3,3'-diaminobenzophenone prepared in diglyme, or 4,4'-oxydianiline prepared in dimethyl acetamide, or 3,3'-diaminodiphenylsulfone prepared in diglyme. Finally, this solution is treated with an organic dehydrating agent. St. Clair found that the solubility of the resulting polyimide in the reaction mixture affects the resulting degree of crystallinity. Also, the time that the dehydrated polymer is in contact with the final solution affects the degree of crystallinity.
An object of the present invention is to provide a direct process for preparing semi-crystalline polyimides directly from a mixture of a dianhydride and a diamine.
Another object of the invention is to provide a direct process for preparing semi-crystalline polyimides which may be processed at temperatures just above their melting points.
Another object of the invention is to form finely divided polyimide particles directly from the reaction mixture.